Magnetic resonance imaging (MRI) is used in medicine to produce images of the internal organs of a patient being examined. In MRI, a static magnetic field is applied to the body of the patient to define an equilibrium axis of magnetic alignment in the region of the body being examined. A radio frequency field is then applied to the region being examined in a direction orthogonal to the static magnetic field direction to excite magnetic resonance in the region. This resonance produces signals in r.f. coils placed adjacent the body. Normally separate coils are used for excitation and detection although the same coil or coils may be used for both purposes. The detected signals are processed to produce signals representing an image of the patient's body and this image is visually displayed. In so-called interventional MRI devices such as catheters, biopsy needles and endoscopes may be physically introduced into the body. The term endoscope is intended to encompass colonoscopes, laparoscopes, cystoscopes and gastroscopes.
The present invention is particularly concerned with the provision of endoscopes which are suitable for use with MRI, such endoscopes having a built-in miniature r.f. coil. Examples of such endoscopes are described in U.S. Pat. No. 4 960 106 and U.S. Pat. No. 5 035 231. It has been proposed in the former patent to make such a coil removable to obtain a better optical field of view when required, but this would cause sealing problems.
In addition, there are a wide variety of MRI systems both in terms of construction and the power of the main magnetic field and therefore, because of the variety of main magnetic fields provided by different designs of MRI equipment each such coil must be tailored/tuned to the appropriate main magnetic field strength. Hitherto, it has been necessary for the MRI equipment user to have a range of particular endoscopes each of which has a different coil to suit a particular main magnetic field strength.